The present invention relates generally to improved formulations for the treatment of open-type evaporative cooling water systems, and more specifically to corrosion inhibiting formulations stable in the presence of strong and highly effective oxidizing microbiocides. These formulations retain their corrosion inhibiting properties for metals having zinc-based surface coatings thereon even when in admixture with oxidizing microbiocides.
Zinc based coatings are commonly employed on surfaces of ferrous metals to create galvanized sheet metal for use in manufacturing components used in open evaporative cooling systems. The well known galvanized metals are a primary example of such zinc based sacrificial coatings.
While stable in the presence of an oxidizing microbiocide, the materials utilized in formulating the present invention are environmentally friendly, and do not lose their effectiveness against the ever present formation or generation of white rust deposit.
Open evaporative cooling water circulation systems which are commonly employed in large industrial or commercial installations require large amounts of cooling water. In the operation of evaporative systems utilizing cooling water, quantities of the water are continually evaporating and lost to the atmosphere, thus creating a need for additional amounts of make-up water to be added at a rate dependent upon the immediate service requirements of the installation. As introduced, make-up water commonly contains a number of impurities and/or contaminants including dissolved gases, dissolved chemical compounds and suspended particulates. Normal operation of these cooling systems results in the consumption of reasonably large quantities of water, primarily through evaporation and this results in a buildup of these contaminants or impurities, which in turn leads to elevated levels or concentrations of these contaminants. Of particular concern is the elevation or increase of carbonate alkalinity and the associated increase in pH levels.
This increase in carbonate alkalinity often results in corrosion of components or parts in the evaporative system, with the zinc based or galvanized coatings present in the systems being highly susceptible. Evidence of corrosion in these coatings typically appears visually as a white, waxy, adherent deposit on the surfaces of the components. This corrosive mechanism or syndrome is commonly referred to as xe2x80x9cWhite Rustxe2x80x9d by those in the water treatment industries. White rust has been identified as the product of a corrosive mechanism involving metallic zinc and carbonate ions with the reaction normally resulting in the formation of the compound ZnCO3.3Zn(OH)2.H2O. White rust corrosion may quickly result in the loss of corrosion inhibition or protection of the ferrous metal substrate due to deterioration or loss of a portion of the zinc coating. If left unchecked, rapid anodic corrosion of zinc coated parts may occur, leading to the premature failure of components present in the cooling system.
There are presently several current methods that have been employed or proposed for the prevention of white rust corrosion; they include the following:
(A) The addition of a sufficient quantity of an acid, most commonly sulfuric acid, to the cooling water in order to adjust the pH downwardly and prevent the creation of or greatly reduce the concentration of carbonate ions in the cooling water. This has been suggested as a way to preclude the formation of zinc carbonate and thereby prevent or inhibit white rust corrosion. The addition of an acid feed to cooling water systems, however, poses safety hazards to those personnel responsible for handling the acid or working with the system, and also has the potential of contributing to aggressive corrosion of metallic parts in the event of an overfeed of acid to the system.
(B) Another approach which has proven capable of reducing or preventing white rust corrosion is the addition of amounts of orthophosphate and/or zinc chemical compounds to the cooling water in order to provide a zinc-orthophosphate film. In such systems, both the orthophosphate and zinc compounds are generally present in quantities of 20 to 100 mg/l as PO4 and as Zn. The phosphate and zinc-phosphate treatments are effective as short-term treatments to provide corrosion protection by forming passivating films on metallic surfaces.
In certain applications, the effectiveness of this approach to protection may be short-lived. Due to the rapid degradation of the protective or passivating film, degradation of the film must be followed by a re-passivation step to prevent localized white rust corrosion. The formation of these passivating films is dependent upon a number of other influencing parameters including features of the cooling equipment being employed and/or the water treatments being utilized. Thus, the quality and longevity of the passivating films remain as either uncertain or indeterminable variables and the systems must be continually monitored for detection of film failure and white rust formation.
Additional disadvantages of such passivating treatments include the possible formation of undesired films or deposits on heat transfer surfaces resulting in decreased equipment efficiency. Concerns for regulatory measures relative to the cooling water disposal are ever present. Potential also exists for damage to the passivating film by oxidizing biocide treatments, by over feed of acidic pH control chemicals and/or by physical erosion.
The use of hard make-up water (water containing ions which contribute to hardness, i.e., calcium and magnesium ions), is recommended in conjunction with a number of current film-forming methods for white rust control. Many of the film-forming corrosion inhibitors which are typically utilized at present require the incorporation of calcium ions. Calcium ions in the cooling water are believed to compete with zinc from the zinc coatings for the carbonate ions, thereby reducing the formation of zinc carbonate.
However, delivering hard water feed to cooling systems is disadvantageous in those instances where the available make-up water contains only modest quantities or no dissolved calcium at all. It is also disadvantageous in those instances where make-up water has been softened in order to prevent or reduce the undesirable calcium carbonate scale formation on heat transfer surfaces.
It will be appreciated that a benign approach to the reduction of white rust corrosion in galvanized or zinc-based coatings capable of circumventing known disadvantages of previous techniques would be a welcome solution to this longstanding problem associated with water cooling systems. This is especially true for a cooling water treatment that would be compatible with the existing water conditions (i.e., relatively high pH and alkalinity and relatively low hardness). Of particular added interest is the utilization of a white rust corrosion inhibitor which is stable in the presence of oxidizing microbiocides such as the commonly utilized bromine, chlorine, ozone, and similar compounds.
Galvanized metal corrosion inhibitor formulations as set forth in U.S. Pat. No. 5,407,597 entitled xe2x80x9cGalvanized Metal Corrosion Inhibitorxe2x80x9d, assigned to the same assignee as the present application, are effective in the prevention of white rust corrosion of galvanized steel surfaces of recirculating evaporative cooling systems operating with cooling water of alkaline pH, and do not require the use of acid to adjust the cooling water pH nor pre-passivation of galvanized metal surfaces as described above.
The inhibitor formulations commonly utilized, including those set forth in U.S. Pat. No. 5,407,597 have been found in practice to be somewhat unstable or incompatible with oxidizing microbiocides commonly used for the prevention of microbiological fouling by algae, bacteria and fungi. The use of oxidizing microbiocides is an integral component of any effective microbiological control program. The prevention of growth of explosive populations of microbes is imperative in controlling: (a) organic fouling or buildup within the system which leads to a reduction in the heat transfer efficiency of the cooling tower system; (b) microbiologically induced corrosion of cooling tower components which reduces the effective life of the cooling tower system; and (c) the potential distribution or spread of disease through human contact with cooling tower water or mist (including such dangerous maladies as Pontiac Fever and Legionnaires Disease). It is therefore, apparent that compatibility and stability of cooling tower inhibitor formulations with oxidizing microbiocides is highly advantageous in the practical treatment of cooling tower systems.
Accordingly, it is a primary object of the present invention to provide cooling water treatment formulations which are capable of preventing or significantly reducing white rust corrosion of zinc coatings utilized in evaporative cooling water systems thereby providing a mechanism for extending the useful life of that equipment.
It is another object of the invention to provide a cooling water treatment system capable of preventing or significantly reducing white rust corrosion of zinc-based coatings present on components used in open evaporative cooling water systems in order to avoid the reduction in efficiency or other limitations created when acidic additives are introduced to the feed water supply.
A further object of the present invention is to provide corrosion inhibiting formulations which are stable in the presence of oxidizing microbiocides and yet are effective in the prevention of white rust corrosion of zinc coatings in cooling systems using water with elevated carbonate alkalinity and pH values, thereby eliminating the need for the addition of a neutralizing acid feed to the water.
A still further object of the present invention is to provide formulations stable in oxidizing microbiocide formulations, and yet effective in the prevention of white rust corrosion of zinc coatings in evaporative cooling water systems employing soft water essentially free of calcium or magnesium ions.
Yet another object of the invention is to provide formulations for inhibitors which are both stable in the presence of oxidizing microbiocides and effective in the prevention of white rust corrosion of zinc coatings in cooling water systems with the chemicals comprising the formulation neither contributing to buildup of scale, creation of deposits, nor corrosion of the evaporative cooling water system when utilized at residuals necessary to provide effective white rust control.
It is yet a further object of the present invention to provide formulations effective in preparing stable working solutions for prevention of white rust corrosion of galvanized or zinc coatings, with solutions from these formulations being compatible with other chemical components typically utilized in cooling water systems for controlling scale and for reducing deposition, inhibiting corrosion and inhibiting microbiological fouling.
It is a still further object of the present invention to provide formulations which are both stable in the presence of oxidizing microbiocides and effective in the prevention and inhibition of white rust corrosion of zinc coatings, while further being effective in inhibiting scale buildup and metallic corrosion, and particularly that of copper corrosion.
Other and further objects of the present invention will become apparent to those skilled in the art upon a study of the following specification and appended claims.